More Thoughts on Integrated Analog

We had a poll posted online recently asking folks about whether they would use an integrated solution for an analog design problem. The results, while unscientific, are food for thought. We need to discuss integrated analog more, because I think there is insufficient understanding of what exactly it is.

The largest number of respondents indicated they would use an integrated solution. Pretty straightforward -- they're in with no reservations. The smallest numbers of respondents said "No." Just above the no votes was "Maybe." So of these bottom two groups, they either need to know more (a hopeful sign) or maybe (I'm hoping) if they just knew more, they wouldn't say no and we could bring them onboard. The one I like the most though is the "Perhaps, if I knew more" response. That is strongly pointing to the need for more education or promotion.

Poll Results: What Does This Mean?

Our recent poll on integrated analog raises some questions.

Integrated analog can mean different things to different people. It could mean programmable analog. Here's a short quote pulled from Wikipedia that describes a form of programmable analog that I find to be accurate:

A field-programmable analog array (FPAA) is an integrated device containing configurable analog blocks (CAB) and interconnects between these blocks. Unlike their digital cousin, the FPGA, the devices tend to be more application driven than general purpose as they may be current mode or voltage mode devices. For voltage mode devices, each block usually contains an operational amplifier in combination with programmable configuration of passive components. The blocks can, for example, act as summers or integrators.

The Wikipedia entry goes on to discuss continuous time and discrete time types of analog building blocks. Here's an example of a continuous time type of FPAA:

A Typical FPAA Device's Internal Circuitry

This simple schematic shows the internal connection options for an FPAA.

You can make different subsystem circuitry out of this by interconnecting the op-amps, resistors, and capacitors. The FPAA hasn't gained wide acceptance. Part of the problem is that as an analog circuit designer, you don't have much choice in the op-amp devices. If your application needs an op-amp with just a little more open loop gain or a little less bias current, you're out of luck.

There are other applications for these FPAAs where op-amp parameters aren't critical. For example, if you needed a subsystem circuit to monitor multiple power supplies and report if any output voltage had drifted above or below a certain point, you would need a window comparator. A window comparator is used to determine if a voltage is within a certain range. It's sort of a Goldilocks circuit -- not too high; not too low; it's just right.

The discrete time types of analog building blocks mentioned in the Wikipedia entry are circuits that have a sample clock and a switched capacitor network (a sample and hold). Here again, the analog engineer may find that what an IC manufacturer provides does not match well with the engineer's needs.

If you want to read a bit more about FPAA, have a look here at a nice piece from EDN from three years ago.

Another use of the term "integrated analog" that we've seen over the last couple of years applies to devices like the analog front end (AFE). An AFE contains the circuitry used to interface with the sensors that are part of, for example, a weigh-scale, an EEG, an EKG, or a gas analyzer. These sensors are typically used in a half- or full-bridge circuit. The circuitry in the AFE is typically composed of op-amps, tightly matched resistors, low-value capacitors (typically part of active filters), analog multiplexers, and some power supply circuitry such as linear voltage regulators. Note that the tight matching on the resistors is necessary to maintain a high common mode rejection ratio (CMRR) that is needed when monitoring a bridge circuit. Since conversion of the data to a digital format is almost always needed, a delta-sigma ADC and a voltage reference are sometimes included on the AFE IC.

The application specific standard product (ASSP) or the application specific integrated circuit (ASIC) are also terms that are used with respect to integrated analog. Such parts may also contain circuitry for monitoring sensors (as well as other functions) as with the AFE.

An important point to keep in mind -- indeed, an important change to one's design philosophy -- is that for the AFE, ASSP, and ASIC, the issue of the specifications for the devices internal to the IC is rendered moot (within reasonable limits). The design engineer need only be concerned with overall performance. In this sense, you become more of a systems engineer.

In this age of high automation, many people categorize FPGA and others as extension of the analog design universe. Why that is true when looking at things broadly, they fail the serious test of analog solutions. There are solutions now like Field Programmable Analog Arrays (FPNA) which uses floating gate devices to make opamps, etc. Yet, for them to work, you need a lot of digital solutions. Because my PIC DSP MCU has some opamps may not mean they are providing analog solutions. The best analog strategy is when the engineer enginees systems at the level of silicon. That is where you get the optimal value in performance.

The adoption of integrated analog strategy for many analog challenges is largely due to the ability to precisely design systems for higher performance. There are things analog solution is not good at but when it is an analog problem, digital only offers a brute-force solution. Notice that in this age when power is premium, an experienced engineer will pursue the analog paradigm because it gives you the best value.